RFC 4233 ISDN Q.921-User Adaptation Layer January 20061. Introduction
In this document, the term Q.921-User refers to an upper layer that
uses the services of Q.921, not the user side of ISDN interface [1].
Examples of the upper layer would be Q.931 and QSIG.
This section describes the need for ISDN Q.921-User Adaptation (IUA)
layer protocol as well as how this protocol shall be implemented.
1.1. Scope
There is a need for Switched Circuit Network (SCN) signaling protocol
delivery from an ISDN Signaling Gateway (SG) to a Media Gateway
Controller (MGC) as described in the Framework Architecture for
Signaling Transport [5]. The delivery mechanism SHOULD meet the
following criteria:
* Support for transport of the Q.921/Q.931 boundary primitives
* Support for communication between Layer Management modules on SG
and MGC
* Support for management of SCTP active associations between SG
and MGC
This document supports both ISDN Primary Rate Access (PRA) as well as
Basic Rate Access (BRA) including the support for both point-to-point
and point-to-multipoint modes of communication. This support
includes Facility Associated Signaling (FAS), Non-Facility Associated
Signaling (NFAS), and NFAS with backup D channel. QSIG adaptation
layer requirements do not differ from Q.931 adaptation layer; hence,
the procedures described in this document are also applicable for a
QSIG adaptation layer. For simplicity, only Q.931 will be mentioned
in the rest of this document.
1.2. Terminology
Application Server (AS) - A logical entity serving a specific
application instance. An example of an Application Server is a MGC
handling the Q.931 and call processing for D channels terminated by
the Signaling Gateways. Practically speaking, an AS is modeled at
the SG as an ordered list of one or more related Application Server
Processes (e.g., primary, secondary, tertiary).
Application Server Process (ASP) - A process instance of an
Application Server. Examples of Application Server Processes are
primary or backup MGC instances.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
Association - An association refers to an SCTP association. The
association will provide the transport for the delivery of Q.921-User
protocol data units and IUA adaptation layer peer messages.
Backhaul - A SG terminates the lower layers of an SCN protocol and
backhauls the upper layer(s) to MGC for call processing. For the
purposes of this document, the SG terminates Q.921 and backhauls
Q.931 to MGC.
Fail-over - The capability to re-route signaling traffic as required
between related ASPs in the event of failure or unavailability of the
currently used ASP (e.g., from primary MGC to backup MGC). Fail-over
also applies upon the return to service of a previously unavailable
process.
Host - The computing platform that the ASP process is running on.
Interface - For the purposes of this document, an interface supports
the relevant ISDN signaling channel. This signaling channel MAY be a
16-kbps D channel for an ISDN BRA as well as 64-kbps primary or
backup D channel for an ISDN PRA. For QSIG, the signaling channel is
a Qc channel.
Interface Identifier - The Interface Identifier identifies the
physical interface at the SG for which the signaling messages are
sent/received. The format of the Interface Identifier parameter can
be text or integer, the values of which are assigned according to
network operator policy. The values used are of local significance
only, coordinated between the SG and ASP. Significance is not
implied across SGs served by an AS.
Layer Management - Layer Management is a nodal function that handles
the inputs and outputs between the IUA layer and a local management
entity.
Network Byte Order - Most significant byte first, a.k.a big endian.
Stream - A stream refers to an SCTP stream: a uni-directional logical
channel established from one SCTP endpoint to another associated SCTP
endpoint, within which all user messages are delivered in sequence
except for those submitted to the un-ordered delivery service.
Q.921-User - Any protocol normally using the services of the ISDN
Q.921 (e.g., Q.931, QSIG, etc.).
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
- sequenced delivery of user messages within multiple streams,
with an option for order-of-arrival delivery of individual user
messages,
- optional multiplexing of user messages into SCTP datagrams,
- network-level fault tolerance through support of multi-homing
at either or both ends of an association,
- resistance to flooding and masquerade attacks, and
- data segmentation to conform to discovered path MTU size.
There are scenarios without redundancy requirements and scenarios in
which redundancy is supported below the transport layer. In these
cases, the SCTP functions above MAY be determined to not be required
and TCP MAY be used as the underlying common transport protocol.
1.3.2. Support for the Management of SCTP Associations between the SG and ASPs
The IUA layer at the SG maintains the availability state of all
dynamically registered remote ASPs, in order to manage the SCTP
associations and the traffic between the SG and ASPs. As well, the
active/inactive states of remote ASP(s) are maintained. Active ASPs
are those currently receiving traffic from the SG.
The IUA layer MAY be instructed by local management to establish an
SCTP association to a peer IUA node. This can be achieved using the
M-SCTP ESTABLISH primitive to request, indicate, and confirm the
establishment of an SCTP association with a peer IUA node.
The IUA layer MAY also need to inform local management of the status
of the underlying SCTP associations using the M-SCTP STATUS request
and indication primitive. For example, the IUA MAY inform local
management of the reason for the release of an SCTP association,
determined either locally within the IUA layer or by a primitive from
the SCTP.
1.3.3. ASP Fail-over Model and Terminology
The IUA layer supports ASP fail-over functions in order to support a
high availability of call processing capability. All Q.921-User
messages incoming to an SG are assigned to a unique Application
Server, based on the Interface Identifier of the message.
The Application Server is, in practical terms, a list of all ASPs
configured to process Q.921-User messages from certain Interface
Identifiers. One or more ASPs in the list are normally active (i.e.,
handling traffic) while any others MAY be unavailable or inactive, to
be possibly used in the event of failure or unavailability of the
active ASP(s).
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
The IUA layer supports an n+k redundancy model (active-standby, load
sharing, broadcast) where n is the minimum number of redundant ASPs
required to handle traffic and k ASPs are available to take over for
a failed or unavailable ASP. Note that 1+1 active/standby redundancy
is a subset of this model. A simplex 1+0 model is also supported as
a subset, with no ASP redundancy.
1.3.4. Client/Server Model
It is recommended that the SG and ASP be able to support both client
and server operation. The peer endpoints using IUA SHOULD be
configured so that one always takes on the role of client and the
other the role of server for initiating SCTP associations. The
default orientation would be for the SG to take on the role of server
while the ASP is the client. In this case, ASPs SHOULD initiate the
SCTP association to the SG.
The SCTP and TCP Registered User Port Number Assignment for IUA is
9900.
1.4. Services Provided by the IUA Layer1.4.1. Support for Transport of Q.921/Q.931 Boundary Primitives
In the backhaul scenario, the Q.921/Q.931 boundary primitives are
exposed. IUA layer needs to support all of the primitives of this
boundary to successfully backhaul Q.931.
This includes the following primitives [1]:
DL-ESTABLISH
The DL-ESTABLISH primitives are used to request, indicate, and
confirm the outcome of the procedures for establishing multiple frame
operation.
DL-RELEASE
DL-RELEASE primitives are used to request, indicate, and confirm the
outcome of the procedures for terminating a previously established
multiple frame operation, or for reporting an unsuccessful
establishment attempt.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
DL-DATA
The DL-DATA primitives are used to request and indicate layer 3
(Q.931) messages that are to be transmitted, or have been received,
by the Q.921 layer using the acknowledged information transfer
service.
DL-UNIT DATA
The DL-UNIT DATA primitives are used to request and indicate layer 3
(Q.931) messages that are to be transmitted, by the Q.921 layer using
the unacknowledged information transfer service.
1.4.2. Support for Communication between Layer Management Modules on SG and MGC
It is envisioned that the IUA layer needs to provide some services
that will facilitate communication between Layer Management modules
on the SG and MGC. These primitives are shown below:
M-TEI STATUS
The M-TEI STATUS primitives are used to request, confirm, and
indicate the status (assigned/unassigned) of an ISDN Terminal
Endpoint Identifier (TEI).
M-ERROR
The M-ERROR primitive is used to indicate an error with a received
IUA message (e.g., interface identifier value is not known to the
SG).
1.4.3. Support for Management of Active Associations between SG and MGC
A set of primitives between the IUA layer and the Layer Management is
defined below to help the Layer Management manage the SCTP
association(s) between the SG and MGC. The IUA layer can be
instructed by the Layer Management to establish an SCTP association
to a peer IUA node. This procedure can be achieved using the M-SCTP
ESTABLISH primitive.
M-SCTP ESTABLISH
The M-SCTP ESTABLISH primitives are used to request, indicate, and
confirm the establishment of an SCTP association to a peer IUA node.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
M-SCTP RELEASE
The M-SCTP RELEASE primitives are used to request, indicate, and
confirm the release of an SCTP association to a peer IUA node.
The IUA layer MAY also need to inform the status of the SCTP
associations to the Layer Management. This can be achieved using the
M-SCTP STATUS primitive.
M-SCTP STATUS
The M-SCTP STATUS primitives are used to request and indicate the
status of the underlying SCTP association(s).
The Layer Management MAY need to inform the IUA layer of an AS/ASP
status (i.e., failure, active, etc.), so that messages can be
exchanged between IUA layer peers to stop traffic to the local IUA
user. This can be achieved using the M-ASP STATUS primitive.
M-ASP STATUS
The ASP status is stored inside IUA layer on both the SG and MGC
sides. The M-ASP STATUS primitive can be used by Layer Management to
request the status of the Application Server Process from the IUA
layer. This primitive can also be used to indicate the status of the
Application Server Process.
M-ASP-UP
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Up message for the Application Server Process. It can also be used
to generate an ASP Up Acknowledgement.
M-ASP-DOWN
The M-ASP-DOWN primitive can be used by Layer Management to send a
ASP Down message for the Application Server Process. It can also be
used to generate an ASP Down Acknowledgement.
M-ASP-ACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Active message for the Application Server Process. It can also be
used to generate an ASP Active Acknowledgement.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
M-ASP-INACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Inactive message for the Application Server Process. It can also be
used to generate an ASP Inactive Acknowledgement.
M-AS STATUS
The M-AS STATUS primitive can be used by Layer Management to request
the status of the Application Server. This primitive can also be
used to indicate the status of the Application Server.
1.5. Functions Implemented by the IUA Layer1.5.1. Mapping
The IUA layer MUST maintain a map of the Interface Identifier to a
physical interface on the Signaling Gateway. A physical interface
would be a T1 line, E1 line, etc., and could include the Time-
Division Multiplexing (TDM) timeslot. In addition, for a given
interface the SG MUST be able to identify the associated signaling
channel. IUA layers on both SG and MGC MAY maintain the status of
ISDN Terminal Endpoint Identifiers (TEIs) and Service Access Point
Identifiers (SAPIs).
The SG maps an Interface Identifier to an SCTP association/stream
only when an ASP sends an ASP Active message for a particular
Interface Identifier. It MUST be noted, however, that this mapping
is dynamic and could change at any time due to a change of ASP state.
This mapping could even temporarily be invalid, for example, during
fail-over of one ASP to another. Therefore, the SG MUST maintain the
states of AS/ASP and reference them during the routing of an messages
to an AS/ASP.
One example of the logical view of relationship between D channel,
Interface Identifier, AS, and ASP in the SG is shown below:
/---------------------------------------------------+
/ /------------------------------------------------|--+
/ / v |
/ / +----+ act+-----+ +-------+ -+--+-|+--+-
D chan1-------->|IID |-+ +-->| ASP |--->| Assoc | v
/ +----+ | +----+ | +-----+ +-------+ -+--+--+--+-
/ +->| AS |--+ Streams
/ +----+ | +----+ stb+-----+
D chan2-------->|IID |-+ | ASP |
+----+ +-----+
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
where IID = Interface Identifier
Note that an ASP can be in more than one AS.
1.5.2. Status of ASPs
The IUA layer on the SG MUST maintain the state of the ASPs it is
supporting. The state of an ASP changes because of reception of
peer-to-peer messages (ASPM messages as described in Section 3.3.2)
or reception of indications from the local SCTP association. ASP
state transition procedures are described in Section 4.3.1.
At a SG, an Application Server list MAY contain active and inactive
ASPs to support ASP load-sharing and fail-over procedures. When, for
example, both a primary and a backup ASP are available, IUA peer
protocol is required to control which ASP is currently active. The
ordered list of ASPs within a logical Application Server is kept
updated in the SG to reflect the active Application Server
Process(es).
Also the IUA layer MAY need to inform the local management of the
change in status of an ASP or AS. This can be achieved using the
M-ASP STATUS or M-AS STATUS primitives.
1.5.3. SCTP Stream Management
SCTP allows a user-specified number of streams to be opened during
the initialization. It is the responsibility of the IUA layer to
ensure proper management of these streams. Because of the
unidirectional nature of streams, an IUA layer is not aware of the
stream number to Interface Identifier mapping of its peer IUA layer.
Instead, the Interface Identifier is in the IUA message header.
The use of SCTP streams within IUA is recommended in order to
minimize transmission and buffering delay, therefore improving the
overall performance and reliability of the signaling elements. It is
recommended that a separate SCTP stream is used for each D channel.
1.5.4. Seamless Network Management Interworking
The IUA layer on the SG SHOULD pass an indication of unavailability
of the IUA-User (Q.931) to the local Layer Management, if the
currently active ASP moves from the ACTIVE state. The Layer
Management could instruct Q.921 to take some action, if it deems
appropriate.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
Parameter Length: 16 bits (unsigned integer)
The Parameter Length field contains the size of the parameter in
bytes, including the Parameter Tag, Parameter Length, and Parameter
Value fields. The Parameter Length does not include any padding
bytes.
Parameter Value: variable-length
The Parameter Value field contains the actual information to be
transferred in the parameter.
The total length of a parameter (including Tag, Parameter Length, and
Value fields) MUST be a multiple of 4 bytes. If the length of the
parameter is not a multiple of 4 bytes, the sender pads the Parameter
at the end (i.e., after the Parameter Value field) with all zero
bytes. The length of the padding is NOT included in the Parameter
Length field. A sender SHOULD NEVER pad with more than 3 bytes. The
receiver MUST ignore the padding bytes.
3.2. IUA Message Header
In addition to the common message header, there will be a specific
message header for QPTM and the TEI Status MGMT messages. The IUA
message header will immediately follow the Common header in these
messages.
This message header will contain the Interface Identifier and Data
Link Connection Identifier (DLCI). The Interface Identifier
identifies the physical interface terminating the signaling channel
at the SG for which the signaling messages are sent/received. The
format of the Interface Identifier parameter can be text or integer.
The Interface Identifiers are assigned according to network operator
policy. The integer values used are of local significance only,
coordinated between the SG and ASP.
The integer-formatted Interface Identifier MUST be supported. The
text-formatted Interface Identifier MAY optionally be supported.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
SPR: Spare 2nd bit in octet 1 (1 bit)
SAPI: Service Access Point Identifier (6 bits)
TEI: Terminal Endpoint Identifier (7 bits)
As an example, SAPI = 0, TEI = 64, SPR = 0 would be encoded as
follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0x5) | Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x0 | 0x81 | 0x0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The DLCI field (including the SAPI and TEI) is coded in accordance
with Q.921.
3.3. IUA Messages
The following section defines the messages and parameter contents.
The IUA messages will use the common message header (Figure 2) and
the IUA message header (Figure 3 and Figure 4).
3.3.1. Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages3.3.1.1. Establish Messages (Request, Confirm, Indication)
The Establish Messages are used to establish a data link on the
signaling channel or to confirm that a data link on the signaling
channel has been established. The MGC controls the state of the D
channel. When the MGC desires the D channel to be in-service, it
will send the Establish Request message.
When the MGC sends an IUA Establish Request message, the MGC MAY
start a timer. This timer would be stopped upon receipt of an IUA
Establish Confirm or Establish Indication. If the timer expires, the
MGC would resend the IUA Establish Request message and restart the
timer. In other words, the MGC MAY continue to request the
establishment of the data link on a periodic basis until the desired
state is achieved or take some other action (notify the Management
Layer).
When the SG receives an IUA Establish Request from the MGC, the SG
shall send the Q.921 Establish Request primitive to the Q.921 entity.
In addition, the SG shall map any response received from the Q.921
entity to the appropriate message to the MGC. For example, if the
Q.921 entity responds with a Q.921 Establish Confirm primitive, the
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
IUA layer shall map this to an IUA Establish Confirm message. As
another example, if the IUA Layer receives a Q.921 Release Confirm or
Release Indication as an apparent response to the Q.921 Establish
Request primitive, the IUA Layer shall map these to the corresponding
IUA Release Confirm or Release Indication messages.
The Establish messages contain the common message header followed by
IUA message header. It does not contain any additional parameters.
3.3.1.2. Release Messages (Request, Indication, Confirmation)
The Release Request message is used to release the data link on the
signaling channel. The Release Confirm and Indication messages are
used to indicate that the data link on the signaling channel has been
released.
If a response to the Release Request message is not received, the MGC
MAY resend the Release Request message. If no response is received,
the MGC can consider the data link as being released. In this case,
signaling traffic on that D channel is not expected from the SG and
signaling traffic will not be sent to the SG for that D channel.
The Release messages contain the common message header followed by
IUA message header. The Release Confirm message is in response to a
Release Request message and it does not contain any additional
parameters. The Release Request and Indication messages contain the
following parameter:
Reason
The format for Release Message parameters is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag (0xf) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
The optional ASP Identifier parameter contains a unique value that is
locally significant among the ASPs that support an AS. The SG should
save the ASP Identifier to be used, if necessary, with the Notify
message (see Section 3.3.3.2).
The optional INFO String parameter can carry any meaningful 8-bit
ASCII [2] character string along with the message. Length of the
INFO String parameter is from 0 to 255 characters. No procedures are
presently identified for its use, but the INFO String MAY be used for
debugging purposes.
3.3.2.2. ASP Up Ack
The ASP Up Ack message is used to acknowledge an ASP Up message
received from a remote IUA peer.
The ASPUP Ack message contains the following parameters:
INFO String (optional)
The format for ASPUP Ack Message parameters is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag = 0x0004 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ INFO String /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format and description of the optional INFO String parameter are
the same as for the ASP Up message (see Section 3.3.2.1).
3.3.2.3. ASP Down (ASPDN)
The ASP Down (ASPDN) message is sent by an ASP to indicate to an SG
that it is NOT ready to receive traffic or maintenance messages.
The ASPDN message contains the following parameters:
INFO String (Optional)
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
Interface Identifiers are being used, the ASP can also send ranges of
Interface Identifiers (Type 0x8). Interface Identifier types Integer
(0x1) and Integer Range (0x8) are allowed in the same message.
Text-formatted Interface Identifiers (0x3) cannot be used with either
Integer (0x1) or Integer Range (0x8) types.
If no Interface Identifiers are included, the message is for all
provisioned Interface Identifiers within the AS or ASes in which the
ASP is provisioned. If only a subset of Interface Identifiers is
included, the ASP is noted as Active for all the Interface
Identifiers provisioned for that AS.
Note: If the optional Interface Identifier parameter is present, the
integer-formatted Interface Identifier MUST be supported, whereas the
text-formatted Interface Identifier MAY be supported.
The format and description of the optional INFO String parameter are
the same as for the ASP Up message (see Section 3.3.2.1.).
An SG that receives an ASPAC with an incorrect Traffic Mode Type for
a particular Interface Identifier will respond with an Error Message
(Cause: Unsupported Traffic Handling Mode).
3.3.2.6. ASP Active Ack
The ASPAC Ack message is used to acknowledge an ASP Active message
received from a remote IUA peer.
The ASPAC Ack message contains the following parameters:
Traffic Mode Type (Mandatory)
Interface Identifier (Optional)
- Combination of integer and integer ranges, OR
- string (text formatted)
INFO String (Optional)
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
The format for the BEAT message is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag = 0x0009 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Heartbeat Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Heartbeat Data parameter contents are defined by the sending
node. The Heartbeat Data could include, for example, a Heartbeat
Sequence Number and/or Timestamp. The receiver of a Heartbeat
message does not process this field as it is only of significance to
the sender. The receiver MUST respond with a Heartbeat Ack message.
3.3.2.10. Heartbeat Ack (BEAT-Ack)
The Heartbeat Ack message is sent in response to a received Heartbeat
message. It includes all the parameters of the received Heartbeat
message, without any change.
3.3.3. Layer Management (MGMT) Messages3.3.3.1. Error (ERR)
The Error message is used to notify a peer of an error event
associated with an incoming message. For example, the message type
might be unexpected given the current state, or a parameter value
might be invalid.
The Error message will have only the common message header. The
Error message contains the following parameters:
Error Code
Diagnostic Information (Optional)
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
The "Unsupported Traffic Handling Mode" error would be sent by an SG
if an ASP sends an ASP Active with an unsupported Traffic Handling
Mode. An example would be a case in which the SG did not support
load-sharing.
The "Unexpected Message" error would be sent by an ASP if it received
a QPTM message from an SG while it was in the Inactive state (the ASP
could optionally drop the message and not send an error). It would
also be sent by an ASP if it received a defined and recognized
message that the SG is not expected to send (e.g., if the MGC
receives an IUA Establish Request message).
The "Protocol Error" error would be sent for any protocol anomaly
(i.e., a bogus message).
The "Invalid Stream Identifier" error would be sent if a message was
received on an unexpected SCTP stream (e.g., a MGMT message was
received on a stream other than "0").
The "Unsupported Interface Identifier Type" error would be sent by an
SG if an ASP sends a text-formatted Interface Identifier and the SG
only supports integer-formatted Interface Identifiers. When the ASP
receives this error, it will need to resend its message with an
integer-formatted Interface Identifier.
The "Unsupported Message Type" error would be sent if a message with
an unexpected or unsupported Message Type is received.
The "Unsupported Message Class" error would be sent if a message with
an unexpected or unsupported Message Class is received.
The "Unassigned TEI" error may be used when the SG receives an IUA
message that includes a TEI that has not been assigned or recognized
for use on the indicated ISDN D-channel.
The "Unrecognized SAPI" error would handle the case of using an SAPI
that is not recognized by the SG. The "Invalid TEI, SAPI
combination" error identifies errors where the TEI is assigned and
the SAPI is recognized, but the combination is not valid for the
interface (e.g., on a Basic Rate Interface (BRI), the MGC tries to
send Q.921 Management messages via IUA when Layer Management at the
SG SHOULD be performing this function).
The "Refused - Management Blocking" error is sent when an ASP Up or
ASP Active message is received and the request is refused for
management reasons (e.g., management lockout).
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
The "ASP Identifier Required" is sent by an SG in response to an ASP
Up message that does not contain an ASP Identifier parameter when the
SG requires one. The ASP SHOULD resend the ASP Up message with an
ASP Identifier.
The "Invalid ASP Identifier" is sent by a SG in response to an ASP Up
message with an invalid (i.e., non-unique) ASP Identifier.
Diagnostic Information: variable length
When included, the optional Diagnostic information can be any
information germane to the error condition, to assist in
identification of the error condition. The Diagnostic information
SHOULD contain the offending message.
Error messages MUST NOT be generated in response to other Error
messages.
3.3.3.2. Notify (NTFY)
The Notify message used to provide an autonomous indication of IUA
events to an IUA peer.
The Notify message will use only the common message header. The
Notify message contains the following parameters:
Status (Mandatory)
ASP Identifier (Optional)
Interface Identifiers (Optional)
INFO String (Optional)
The format for the Notify message with integer-formatted Interface
Identifiers is as follows:
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
1 reserved
2 Application Server Inactive (AS-INACTIVE)
3 Application Server Active (AS-ACTIVE)
4 Application Server Pending (AS-PENDING)
These notifications are sent from an SG to an ASP upon a change in
status of a particular Application Server. The value reflects the
new state of the Application Server.
If the Status Type is Other, then the following Status Information
values are defined:
Value Description
1 Insufficient ASP resources active in AS
2 Alternate ASP Active
3 ASP Failure
These notifications are not based on the SG reporting the state
change of an ASP or AS. In the Insufficient ASP Resources case, the
SG is indicating to an ASP-INACTIVE ASP(s) in the AS that another ASP
is required in order to handle the load of the AS (Load-sharing
mode). For the Alternate ASP Active case, an ASP is informed when an
alternate ASP transitions to the ASP-ACTIVE state in Over-ride mode.
The ASP Identifier (if available) of the Alternate ASP MUST be placed
in the message. For the ASP Failure case, the SG is indicating to
ASP(s) in the AS that one of the ASPs has transitioned to ASP-DOWN.
The ASP Identifier (if available) of the failed ASP MUST be placed in
the message.
The format and description of the optional ASP Identifier are the
same as for the ASP Up message (see Section 3.3.2.1). The format and
description of the optional Interface Identifiers and INFO String
parameters are the same as for the ASP Active message (see Section3.3.2.5).
3.3.3.3. TEI Status Messages (Request, Confirm, and Indication)
The TEI Status messages are exchanged between IUA layer peers to
request, confirm, and indicate the status of a particular TEI.
The TEI Status messages contain the common message header followed by
IUA message header. The TEI Status Request message does not contain
any additional parameters.
In the integrated ISDN Layer 2/3 model (e.g., in traditional ISDN
switches), it is assumed that the Layer Management for the Q.921
Layer and the Q.931 layer are co-located. When backhauling ISDN,
this assumption is not necessarily valid. The TEI Status messages
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
allow the two Layer Management entities to communicate the status of
the TEI. In addition, knowing that a TEI is in service allows the
ASP to request the SG to establish the datalink to the terminal (via
the IUA Establish message) for signaling if the ASP wants to be in
control of data link establishment. Another use of the TEI Status
procedure is where the Layer Management at the ASP can prepare for
send/receive signaling to/from a given TEI and confirm/verify the
establishment of a datalink to that TEI. For example, if a datalink
is established for a TEI that the ASP did not know was assigned, the
ASP can check to see whether it was assigned or whether there was an
error in the signaling message. Also, knowing that a TEI is out of
service, the ASP need not request the SG to establish a datalink to
that TEI.
The TEI Status Indication and Confirm messages contain the following
parameter:
STATUS
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag = 0x0010 | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Status |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The valid values for Status are shown in the following table.
Define Value Description
ASSIGNED 0x0 TEI is considered assigned by Q.921
UNASSIGNED 0x1 TEI is considered unassigned by Q.921
3.3.3.4. TEI Query Message (Request)
The TEI Query message is sent by the ASP to query the TEI(s). This
message consists of the common header and IUA header. The DLCI in
the IUA header MUST be ignored by the SG. The SG will respond to
this message with TEI Status Indication(s).
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RFC 4233 ISDN Q.921-User Adaptation Layer January 20064. Procedures
The IUA layer needs to respond to various primitives it receives from
other layers as well as messages it receives from the peer IUA layer.
This section describes various procedures involved in response to
these events.
4.1. Procedures to Support Service in Section 1.4.1
These procedures achieve the IUA layer's "Transport of Q.921/Q.931
boundary primitives" service.
4.1.1. Q.921 or Q.931 Primitives Procedures
On receiving these primitives from the local layer, the IUA layer
will send the corresponding QPTM message (Data, Unit Data, Establish,
Release) to its peer. While doing so, the IUA layer needs to fill
various fields of the common and specific headers correctly. In
addition, the message needs to be sent on the SCTP stream that
corresponds to the D channel (Interface Identifier).
4.1.2. QPTM Message Procedures
On receiving QPTM messages from a peer IUA layer, the IUA layer on an
SG or MGC needs to invoke the corresponding layer primitives
(DL-ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921
or Q.931 layer.
4.2. Procedures to Support Service in Section 1.4.2
These procedures achieve the IUA layer's "Support for Communication
between Layer Managements" service.
4.2.1. Layer Management Primitives Procedures
On receiving these primitives from the local Layer Management, the
IUA layer will provide the appropriate response primitive across the
internal local Layer Management interface.
An M-SCTP ESTABLISH request from Layer Management will initiate the
establishment of an SCTP association. An M-SCTP ESTABLISH confirm
will be sent to Layer Management when the initiated association setup
is complete. An M-SCTP ESTABLISH indication is sent to Layer
Management upon successful completion of an incoming SCTP association
setup from a peer IUA node.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
An M-SCTP RELEASE request from Layer Management will initiate the
teardown of an SCTP association. An M-SCTP RELEASE confirm will be
sent by Layer Management when the association teardown is complete.
An M-SCTP RELEASE indication is sent to Layer Management upon
successful teardown of an SCTP association initiated by a peer IUA.
M-SCTP STATUS request and indication support a Layer Management query
of the local status of a particular SCTP association.
M-NOTIFY indication and M-ERROR indication indicate to Layer
Management the notification or error information contained in a
received IUA Notify or Error message, respectively. These
indications can also be generated based on local IUA events.
M-ASP STATUS request/indication and M-AS-STATUS request/indication
support a Layer Management query of the local status of a particular
ASP or AS. No IUA peer protocol is invoked.
M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request, and
M-ASP-ACTIVE request allow Layer Management at an ASP to initiate
state changes. These requests result in outgoing IUA ASP UP, ASP
DOWN, ASP INACTIVE, and ASP ACTIVE messages.
M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE
confirmation, and M-ASP-ACTIVE confirmation indicate to Layer
Management that the previous request has been confirmed.
Upon receipt of an M-TEI Status primitive from Layer Management, the
IUA will send the corresponding MGMT message (TEI Status) to its
peer. While doing so, the IUA layer needs to fill various fields of
the common and specific headers correctly.
All MGMT messages are sent on a sequenced stream to ensure ordering.
SCTP stream '0' SHOULD be used.
4.2.2. Receipt of IUA Peer Management Messages
Upon receipt of IUA Management messages, the IUA layer MUST invoke
the corresponding Layer Management primitive indications (e.g., M-AS
Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATUS) to the
local layer management.
M-NOTIFY indication and M-ERROR indication indicate to Layer
Management the notification or error information contained in a
received IUA Notify or Error message. These indications can also be
generated based on local IUA events.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
All MGMT messages are sent on a sequenced stream to ensure ordering.
SCTP stream '0' SHOULD be used.
4.3. Procedures to Support Service in Section 1.4.3
These procedures achieve the IUA layer's "Support for management of
active associations between SG and MGC" service.
4.3.1. AS and ASP State Maintenance
The IUA layer on the SG needs to maintain the states of each ASP as
well as the state of the AS.
4.3.1.1. ASP States
The state of the each ASP, in each AS that it is configured, is
maintained in the IUA layer on the SG. The state of an ASP changes
due to the following type of events:
* Reception of messages from peer IUA layer at that ASP
* Reception of some messages from the peer IUA layer at other
ASPs in the AS
* Reception of indications from SCTP layer
* Local Management intervention
The ASP state transition diagram is shown in Figure 6. The possible
states of an ASP are the following:
ASP-DOWN: Application Server Process is unavailable and/or the
related SCTP association is down. Initially, all ASPs will be in
this state. An ASP in this state SHOULD NOT be sent any IUA
messages.
ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
related SCTP association is up) but application traffic is stopped.
In this state, the ASP can be sent any non-QPTM IUA messages (except
for TEI Status messages).
ASP-ACTIVE: The remote IUA peer at the ASP is available and
application traffic is active.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
The possible states of an AS are the following:
AS-DOWN: The Application Server is unavailable. This state implies
that all related ASPs are in the ASP-DOWN state for this AS.
Initially, the AS will be in this state.
AS-INACTIVE: The Application Server is available but no application
traffic is active (i.e., one or more related ASPs are in the
ASP-INACTIVE state, but none in the ASP-ACTIVE state). The recovery
timer T(r) is not running or has expired.
AS-ACTIVE: The Application Server is available and application
traffic is active. This state implies that at least one ASP is in
the ASP-ACTIVE state.
AS-PENDING: An active ASP has transitioned from active to inactive or
down and it was the last remaining active ASP in the AS. A recovery
timer T(r) will be started and all incoming SCN messages will be
queued by the SG. If an ASP becomes active before T(r) expires, the
AS will move to AS-ACTIVE state and all the queued messages will be
sent to the active ASP.
If T(r) expires before an ASP becomes active, the SG stops queuing
messages and discards all previously queued messages. The AS will
move to AS-INACTIVE if at least one ASP is in ASP-INACTIVE state,
otherwise it will move to AS-DOWN state.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
Once the SCTP association is established and assuming that the local
IUA-User is ready, the local ASP IUA Application Server Process
Maintenance (ASPM) function will initiate the ASPM procedures, using
the ASP Up/-Down/-Active/-Inactive messages to convey the ASP state
to the SG (see Section 4.3.3).
The Layer Management and the IUA layer on SG can communicate the
status of the application server using the M-AS_STATUS primitives.
The Layer Management and the IUA layer on both the SG and ASP can
communicate the status of an SCTP association using the M-SCTP_STATUS
primitives.
If the Layer Management on SG or ASP wants to bring down an SCTP
association for management reasons, it would send M-SCTP RELEASE
request primitive to the local IUA layer. The IUA layer would
release the SCTP association and upon receiving the SCTP-
COMMUNICATION_DOWN indication from the underlying SCTP layer, it
would inform the local Layer Management using M-SCTP_RELEASE confirm
primitive.
If the IUA layer receives an SCTP-COMMUNICATION_DOWN indication from
the underlying SCTP layer, it will inform the Layer Management by
invoking the M-SCTP RELEASE indication primitive. The state of the
ASP will be moved to "Down" at both the SG and ASP.
At an ASP, the Layer Management MAY try to reestablish the SCTP
association using M-SCTP_ESTABLISH request primitive.
In the case of an SCTP-RESTART indication at an ASP, the ASP is now
considered by its IUA peer to be in the ASP-DOWN state. The ASP, if
it is to recover, must begin any recovery with the ASP Up procedure.
4.3.3. ASPM Procedures for Peer-to-Peer Messages
All ASPM messages are sent on a sequenced stream to ensure ordering.
SCTP stream '0' SHOULD be used.
4.3.3.1. ASP Up Procedures
After an ASP has successfully established an SCTP association to an
SG, the SG waits for the ASP to send an ASP Up message, indicating
that the ASP IUA peer is available. The ASP is always the initiator
of the ASP Up message. This action MAY be initiated at the ASP by an
M-ASP_UP request primitive from Layer Management or MAY be initiated
automatically by an IUA management function.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
When an ASP Up message is received at an SG and internally the remote
ASP is in the ASP-DOWN state and not considered locked out for local
management reasons, the SG marks the remote ASP in the state
ASP-INACTIVE and informs Layer Management with an M-ASP_Up indication
primitive. If the SG is aware, via current configuration data, which
Application Servers the ASP is configured to operate in, the SG
updates the ASP state to ASP-INACTIVE in each AS that it is a member.
Alternatively, the SG may move the ASP into a pool of Inactive ASPs
available for future configuration within Application Server(s),
determined in a subsequent ASP Active procedure. If the ASP Up
message contains an ASP Identifier, the SG should save the ASP
Identifier for that ASP. The SG MUST send an ASP Up Ack message in
response to a received ASP Up message even if the ASP is already
marked as ASP-INACTIVE at the SG.
If for any local reason (e.g., management lockout) the SG cannot
respond with an ASP Up Ack message, the SG responds to an ASP Up
message with an Error message with reason "Refused - Management
Blocking".
At the ASP, the ASP Up Ack message received is not acknowledged.
Layer Management is informed with an M-ASP_UP confirm primitive.
When the ASP sends an ASP Up message, it starts timer T(ack). If the
ASP does not receive a response to an ASP Up message within T(ack),
the ASP MAY restart T(ack) and resend ASP Up messages until it
receives an ASP Up Ack message. T(ack) is provisionable, with a
default of 2 seconds. Alternatively, retransmission of ASP Up
messages MAY be put under control of Layer Management. In this
method, expiry of T(ack) results in an M-ASP_UP confirm primitive
carrying a negative indication.
The ASP must wait for the ASP Up Ack message before sending any other
IUA messages (e.g., ASP Active). If the SG receives any other IUA
messages before an ASP Up message is received (other than ASP Down;
see Section 4.3.3.2), the SG MAY discard them.
If an ASP Up message is received and internally the remote ASP is in
the ASP-ACTIVE state, an ASP Up Ack message is returned, as well as
an Error message ("Unexpected Message"), and the remote ASP state is
changed to ASP-INACTIVE in all relevant Application Servers.
If an ASP Up message is received and internally the remote ASP is
already in the ASP-INACTIVE state, an ASP Up Ack message is returned
and no further action is taken.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 20064.3.3.2. ASP Down Procedures
The ASP will send an ASP Down message to an SG when the ASP wishes to
be removed from the list of ASPs in all Application Servers that it
is a member and no longer receive any IUA QPTM or ASPTM messages.
This action MAY be initiated at the ASP by an M-ASP_DOWN request
primitive from Layer Management or MAY be initiated automatically by
an IUA management function.
Whether the ASP is permanently removed from an AS is a function of
configuration management.
The SG marks the ASP as ASP-DOWN, informs Layer Management with an
M-ASP_Down indication primitive, and returns an ASP Down Ack message
to the ASP.
The SG MUST send an ASP Down Ack message in response to a received
ASP Down message from the ASP even if the ASP is already marked as
ASP-DOWN at the SG.
At the ASP, the ASP Down Ack message received is not acknowledged.
Layer Management is informed with an M-ASP_DOWN confirm primitive.
If the ASP receives an ASP Down Ack without having sent an ASP Down
message, the ASP should now consider itself as in the ASP-DOWN state.
If the ASP was previously in the ASP-ACTIVE or ASP-INACTIVE state,
the ASP should then initiate procedures to return itself to its
previous state.
When the ASP sends an ASP Down message, it starts timer T(ack). If
the ASP does not receive a response to an ASP Down message within
T(ack), the ASP MAY restart T(ack) and resend ASP Down messages until
it receives an ASP Down Ack message. T(ack) is provisionable, with a
default of 2 seconds. Alternatively, retransmission of ASP Down
messages MAY be put under control of Layer Management. In this
method, expiry of T(ack) results in an M-ASP_DOWN confirm primitive
carrying a negative indication.
4.3.3.3. IUA Version Control
If a ASP Up message with an unsupported version is received, the
receiving end responds with an Error message, indicating the version
the receiving node supports and notifies Layer Management.
This is useful when protocol version upgrades are being performed in
a network. A node upgraded to a newer version SHOULD support the
older versions used on other nodes it is communicating with. Because
ASPs initiate the ASP Up procedure it is assumed that the Error
message would normally come from the SG.
Morneault, et al. Standards Track [Page 54]

RFC 4233 ISDN Q.921-User Adaptation Layer January 20064.3.3.4. ASP Active Procedures
Any time after the ASP has received an ASP Up Ack from the SG, the
ASP sends an ASP Active message to the SG indicating that the ASP is
ready to start processing traffic. This action MAY be initiated at
the ASP by an M-ASP_ACTIVE request primitive from Layer Management or
MAY be initiated automatically by an IUA management function. In the
case where an ASP is configured/registered to process the traffic for
more than one Application Server across an SCTP association, the
ASPAC contains one or more Interface Identifiers to indicate for
which Application Servers the ASPAC applies.
If the Application Server can be successfully activated, the SG
responds to the ASP with an ASPAC Ack message acknowledging that the
ASPAC message was received and starts sending traffic for the
Application Server to that ASP.
In the case where an "out-of-the-blue" ASP Active message is received
(i.e., the ASP has not registered with the SG or the SG has no static
configuration data for the ASP), the message MAY be silently
discarded.
The SG MUST send an ASP Active Ack message in response to a received
ASP Active message from the ASP, if the ASP is already marked in the
ASP-ACTIVE state at the SG.
At the ASP, the ASP Active Ack message received is not acknowledged.
Layer Management is informed with an M-ASP_ACTIVE confirm primitive.
It is possible for the ASP to receive Data message(s) before the ASP
Active Ack message as the ASP Active Ack and Data messages from an SG
may be sent on different SCTP streams. Message loss is possible as
the ASP does not consider itself in the ASP-ACTIVE state until
reception of the ASP Active Ack message.
When the ASP sends an ASP Active message, it starts timer T(ack). If
the ASP does not receive a response to an ASP Active message within
T(ack), the ASP MAY restart T(ack) and resend ASP Active messages
until it receives an ASP Active Ack message. T(ack) is
provisionable, with a default of 2 seconds. Alternatively,
retransmission of ASP Active messages MAY be put under control of
Layer Management. In this method, expiry of T(ack) results in an M-
ASP_ACTIVE confirm primitive carrying a negative indication.
The ASP MUST wait for the ASP Active Ack message from the SG before
sending any Data messages or it will risk message loss. If the SG
receives QPTM messages before an ASP Active is received, the SG
SHOULD discard these messages.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
There are two modes of Application Server traffic handling in the SG
IUA: Over-ride and Load-sharing. The Type parameter in the ASPAC
message indicates the mode used in a particular Application Server.
If the SG determines that the mode indicates in an ASPAC is
incompatible with the traffic handling mode currently used in the AS,
the SG responds with an Error message indicating Unsupported Traffic
Handling Mode.
In the case of an Over-ride mode AS, reception of an ASPAC message at
an SG causes the redirection of all traffic for the AS to the ASP
that sent the ASPAC. The SG responds to the ASPAC with an ASP Active
Ack message to the ASP. Any previously active ASP in the AS is now
considered Inactive and will no longer receive traffic from the SG
within the AS. The SG sends a Notify (Alternate ASP Active) to the
previously active ASP in the AS, after stopping all traffic to that
ASP.
In the case of a load-share mode AS, reception of an ASPAC message at
an SG causes the direction of traffic to the ASP sending the ASPAC,
in addition to all the other ASPs that are currently active in the
AS. The algorithm at the SG for load-sharing traffic within an AS to
all the active ASPs is implementation dependent. The algorithm
could, for example, be round-robin or based on information in the
Data message, such as Interface Identifier, depending on the
requirements of the application and the call state handling
assumptions of the collection of ASPs in the AS. The SG responds to
the ASPAC with an ASP Active Ack message to the ASP.
4.3.3.5. ASP Inactive Procedures
When an ASP wishes to withdraw from receiving traffic within an AS,
the ASP sends an ASP Inactive message to the SG. This action MAY be
initiated at the ASP by an M-ASP_INACTIVE request primitive from
Layer Management or MAY be initiated automatically by an IUA
management function. In the case where an ASP is configured/
registered to process the traffic for more than one Application
Server across an SCTP association, the ASPIA contains one or more
Interface Identifiers to indicate for which Application Servers the
ASP Inactive message applies.
There are two modes of Application Server traffic handling in the SG
IUA when withdrawing an ASP from service: Over-ride and Load-sharing.
In the case of an Over-ride mode AS, where normally another ASP has
already taken over the traffic within the AS with an Over-ride ASPAC
message, the ASP that sends the ASPIA message is already considered
by the SG to be ASP-INACTIVE. An ASPIA Ack message is sent to the
ASP, after ensuring that all traffic is stopped to the ASP.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
In the case of a Load-share mode AS, the SG moves the ASP to the
ASP-INACTIVE state and the AS traffic is re-allocated across the
remaining ASP-ACTIVE ASPs per the load-sharing algorithm currently
used within the AS. An ASPIA Ack message is sent to the ASP after
all traffic is halted to the ASP. A Notify (Insufficient ASPs)
message MAY be sent to all inactive ASPs, if required.
When the ASP sends an ASP Inactive message it starts timer T(ack).
If the ASP does not receive a response to an ASP Inactive message
within T(ack), the ASP MAY restart T(ack) and resend ASP Inactive
messages until it receives an ASP Inactive Ack message. T(ack) is
provisionable, with a default of 2 seconds. Alternatively,
retransmission of ASP Inactive messages MAY be put under control of
Layer Management. In this method, expiry of T(ack) results in a M-
ASP_Inactive confirm primitive carrying a negative indication.
If no other ASPs in the Application Server are in the state
ASP-ACTIVE, the SG MUST send a Notify ("AS-Pending") message to all
of the ASPs in the AS that are in the state ASP-INACTIVE. The SG
SHOULD start buffering the incoming messages for T(r) seconds, after
which messages MAY be discarded. T(r) is configurable by the network
operator. If the SG receives an ASP Active message from an ASP in
the AS before expiry of T(r), the buffered traffic is directed to
that ASP and the timer is cancelled. If T(r) expires, the AS is
moved to the AS-INACTIVE state.
At the ASP, the ASP Inactive Ack message received is not
acknowledged. Layer Management is informed with an M-ASP_INACTIVE
confirm primitive. If the ASP receives an ASP Inactive Ack without
having sent an ASP Inactive message, the ASP should now consider
itself as in the ASP-INACTIVE state. If the ASP was previously in
the ASP-ACTIVE state, the ASP should then initiate procedures to
return itself to its previous state.
4.3.3.6. Notify Procedures
A Notify message reflecting a change in the AS state MUST be sent to
all ASPs in the AS, except those in the ASP-DOWN state, with
appropriate Status Information and any ASP Identifier of the failed
ASP. At the ASP, Layer Management is informed with an M-NOTIFY
indication primitive. The Notify message must be sent whether the AS
state change was a result of an ASP failure or reception of an ASP
State Management (ASPSM) / ASP Traffic Management (ASPTM) message.
In the second case, the Notify message MUST be sent after any related
acknowledgement messages (e.g., ASP Up Ack, ASP Down Ack, ASP Active
Ack, or ASP Inactive Ack).
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
In the case where a Notify ("AS-Pending") message is sent by an SG
that now has no ASPs active to service the traffic, or a NTFY
("Insufficient ASPs") is sent in the Load-share mode, the Notify does
not explicitly compel the ASP(s) receiving the message to become
active. The ASPs remain in control of what (and when) action is
taken.
4.3.3.7. Heartbeat
The optional Heartbeat procedures MAY be used when operating over
transport layers that do not have their own heartbeat mechanism for
detecting loss of the transport association (i.e., other than the
SCTP).
Either IUA peer may optionally send Heartbeat messages periodically,
subject to a provisionable timer T(beat). Upon receiving a Heartbeat
message, the IUA peer MUST respond with a Heartbeat Ack message.
If no Heartbeat Ack message (or any other IUA message) is received
from the IUA peer within 2*T(beat), the remote IUA peer is considered
unavailable. Transmission of Heartbeat messages is stopped and the
signaling process SHOULD attempt to re-establish communication if it
is configured as the client for the disconnected IUA peer.
The BEAT message MAY optionally contain an opaque Heartbeat Data
parameter that MUST be echoed back unchanged in the related Beat Ack
message. The ASP upon examining the contents of the returned BEAT
Ack message MAY choose to consider the remote ASP as unavailable.
The contents/format of the Heartbeat Data parameter is implementation
dependent and only of local interest to the original sender. The
contents MAY be used, for example, to support a Heartbeat sequence
algorithm (to detect missing Heartbeats), and/or a timestamp
mechanism (to evaluate delays).
Note: Heartbeat-related events are not shown in Figure 6, "ASP State
Transition Diagram".
5. Examples5.1. Establishment of Association and Traffic between SGs and ASPs5.1.1. Single ASP in an Application Server (1+0 sparing)
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and an ASP, where only one ASP
is configured within an AS (no backup). It is assumed that the SCTP
association is already setup.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 20065.2. ASP Traffic Fail-over Examples5.2.1. (1+1 Sparing, withdrawal of ASP, Backup Over-ride)
The following example shows a case in which an ASP withdraws from
service:
SG ASP1 ASP2
| | |
|<-----ASP Inactive-------| |
|----ASP Inactive Ack---->| |
| | |
|----NTFY(AS-Pending)---->| |
|-------------------NTFY(AS-Pending)---------------->|
| | |
|<------------------------------ ASP Active----------|
|-----------------------------ASP Active Ack)------->|
| | |
|----NTFY(AS-ACTIVE)----->| |
|-------------------NTFY(AS-ACTIVE)----------------->|
In this case, the SG notifies ASP2 that the AS has moved to the Down
state. The SG could have also (optionally) sent a Notify message
when the AS moved to the Pending state.
Note: If the SG detects loss of the IUA peer (IUA heartbeat loss or
detection of SCTP failure), the initial SG-ASP1 ASP Inactive message
exchange would not occur.
5.2.2. (1+1 Sparing, Backup Over-ride)
The following example shows a case in which ASP2 wishes to override
ASP1 and take over the traffic:
SG ASP1 ASP2
| | |
|<-------------------------------ASP Active----------|
|-----------------------------ASP Active Ack-------->|
|----NTFY( Alt ASP-Act)-->|
| | |
In this case, the SG notifies ASP1 that an alternative ASP has
overridden it.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 20065.2.3. (n+k Sparing, Load-sharing case, withdrawal of ASP)
Following on from the example in Section 5.1.4, and ASP1 withdraws
from service:
SG ASP1 ASP2 ASP3
| | | |
|<----ASP Inact------| | |
|---ASP Inact Ack--->| | |
| | | |
|---------------------------------NTFY(Ins. ASPs)----------->|
| | | |
|<-----------------------------------------ASP Act (Ldshr)---|
|-------------------------------------------ASP Act (Ack)--->|
| | | |
In this case, the SG has knowledge of the minimum ASP resources
required (implementation dependent), for example, if the SG knows
that n+k = 2+1 for a load-share AS and n currently equals 1.
Note: If the SG detects loss of the ASP1 IUA peer (IUA heartbeat
loss or detection of SCTP failure), the first SG-ASP1 ASP Inactive
message exchange would not occur.
5.3. Q.921/Q.931 Primitives Backhaul Examples
When the IUA layer on the ASP has a QPTM message to send to the SG,
it will do the following:
- Determine the correct SG
- Find the SCTP association to the chosen SG
- Determine the correct stream in the SCTP association based on
the D channel
- Fill in the QPTM message, fill in IUA Message Header, fill in
Common Header
- Send the QPTM message to the remote IUA peer in the SG, over
the SCTP association
When the IUA layer on the SG has a QPTM message to send to the ASP,
it will do the following:
- Determine the AS for the Interface Identifier
- Determine the Active ASP (SCTP association) within the AS
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
- Determine the correct stream in the SCTP association based on
the D channel
- Fill in the QPTM message, fill in IUA Message Header, fill in
Common Header
- Send the QPTM message to the remote IUA peer in the ASP, over
the SCTP association
An example of the message flows for establishing a data link on a
signaling channel, passing PDUs and releasing a data link on a
signaling channel is shown below. An active association between MGC
and SG is established (Section 5.1) prior to the following message
flows.
SG ASP
<----------- Establish Request
Establish Confirm ---------->
<----------- Data Request
Data Indication ----------->
<----------- Data Request
Data Indication ----------->
<----------- Data Request
<----------- Data Request
Data Indication ----------->
<----------- Release Request (RELEASE_MGMT)
Release Confirm ---------->
An example of the message flows for a failed attempt to establish a
data link on the signaling channel is shown below. In this case, the
gateway has a problem with its physical connection (e.g., Red Alarm),
so it cannot establish a data link on the signaling channel.
SG ASP
<----------- Establish Request (ESTABLISH_START)
Release Indication ---------->
(RELEASE_PHYS)
5.4. Layer Management Communication Examples
An example of the message flows for communication between Layer
Management modules between SG and ASP is shown below. An active
association between ASP and SG is established (Section 5.1) prior to
the following message flows.
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SG ASP
<----------- Data Request
Error Indication ---------->
(INVALID_TEI)
<----------- TEI Status Request
TEI Status Confirm ---------->
(Unassigned)
6. Security
The security considerations discussed in "Security Considerations for
SIGTRAN Protocols", RFC 3788 [3], apply to this document.
7. IANA Considerations7.1. SCTP Payload Protocol Identifier
The IANA has assigned an IUA value for the Payload Protocol
Identifier in SCTP Payload Data chunk. The following SCTP Payload
Protocol Identifier has been registered:
IUA "1"
The SCTP Payload Protocol Identifier is included in each SCTP Data
chunk, to indicate which protocol the SCTP is carrying. This Payload
Protocol Identifier is not directly used by SCTP but MAY be used by
certain network entities to identify the type of information being
carried in a Data chunk.
The User Adaptation peer MAY use the Payload Protocol Identifier as a
way of determining additional information about the data being
presented to it by SCTP.
7.2. IUA Protocol Extensions
This protocol may also be extended through IANA in three ways:
-- through definition of additional message classes,
-- through definition of additional message types, and
-- through definition of additional message parameters.
The definition and use of new message classes, types, and parameters
are an integral part of SIGTRAN adaptation layers. Thus, these
extensions are assigned by IANA through an IETF Consensus action as
defined in [7].
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The proposed extension must in no way adversely affect the general
working of the protocol.
7.2.1. IETF-Defined Message Classes
The documentation for a new message class MUST include the following
information:
(a) A long and short name for the message class.
(b) A detailed description of the purpose of the message class.
7.2.2. IETF-Defined Message Types
Documentation of the message type MUST contain the following
information:
(a) A long and short name for the new message type.
(b) A detailed description of the structure of the message.
(c) A detailed definition and description of intended use of each
field within the message.
(d) A detailed procedural description of the use of the new
message type within the operation of the protocol.
(e) A detailed description of error conditions when receiving this
message type.
When an implementation receives a message type that it does not
support, it MUST respond with an Error (ERR) message with an Error
Code of Unsupported Message Type.
7.2.3. IETF-Defined TLV Parameter Extension
Documentation of the message parameter MUST contain the following
information:
(a) Name of the parameter type.
(b) Detailed description of the structure of the parameter field.
This structure MUST conform to the general type-length-value
format described in Section 3.1.5.
(c) Detailed definition of each component of the parameter value.
(d) Detailed description of the intended use of this parameter type,
and an indication of whether and under what circumstances
multiple instances of this parameter type may be found within the
same message type.
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[8] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002.
11. Change Log
Below is a list of the major changes between this document and RFC3057.
1. The TEI Query message was added.
2. An explanation of the DLCI format (shown in Figure 6) is
provided.
3. Aligned the ASP and AS procedures in Section 4 with RFC3331 and
RFC3332.
4. Alinged the format of the ASPSM and ASPTM messages with RFC3331
and RFC3332. These changes include removing the Reason field
from the ASP Down and ASP Down Ack messages and the Traffic Mode
Type field from the ASP Inactive and ASP Inactive Ack messages.
5. Sections 1.3.3 and 1.3.4 were moved to Appendix A. A new section
was added in place of Section 1.3.3.
6. The references have been split between Normative and Informative.
7. The new Sigtran security document is referenced and Section 6 has
been updated appropriately.
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Appendix A
A.1. Signaling Network Architecture
A Signaling Gateway is used to support the transport of Q.921-User
signaling traffic to one or more distributed ASPs (e.g., MGCs).
Clearly, the IUA protocol is not designed to meet the performance and
reliability requirements for such transport by itself. However, the
conjunction of distributed architecture and redundant networks does
allow for a sufficiently reliable transport of signaling traffic over
IP. The IUA protocol is flexible enough to allow its operation and
management in a variety of physical configurations, enabling Network
Operators to meet their performance and reliability requirements.
To meet the ISDN signaling reliability and performance requirements
for carrier grade networks, Network Operators SHOULD ensure that
there is no single point of failure provisioned in the end-to-end
network architecture between an ISDN node and an IP ASP.
Depending of course on the reliability of the SG and ASP functional
elements, this can typically be met by the provision of redundant
Quality of Service (QoS)-bounded IP network paths for SCTP
Associations between SCTP End Points, and redundant Hosts, and
redundant SGs. The distribution of ASPs within the available Hosts
is also important. For a particular Application Server, the related
ASPs SHOULD be distributed over at least two Hosts.
An example logical network architecture relevant to carrier-grade
operation in the IP network domain is shown in Figure 8 below:
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In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
message for the Interface Identifiers registered. ASP1 in Host2
would normally be brought to the active state upon failure of, or
loss of connectivity to, ASP1/Host1. In this example, both ASPs are
Up, meaning that the related SCTP association and far-end IUA peer
are ready.
The AS List at SG1 might also be set up in load-share mode as shown
below:
Interface Identifier(s) - Application Server #1
ASP1/Host1 - State=Up, Active
ASP1/Host2 - State=Up, Active
In this case, both the ASPs would be sent a portion of the traffic.
In the process of fail-over, it is recommended that in the case of
ASPs supporting call processing, stable calls do not get released.
It is possible that calls in transition MAY fail, although measures
of communication between the ASPs involved can be used to mitigate
this problem. For example, the two ASPs MAY share call state via
shared memory, or MAY use an ASP-to-ASP protocol to pass call state
information. The ASP-to-ASP protocol is outside the scope of this
document.
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RFC 4233 ISDN Q.921-User Adaptation Layer January 2006
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